Mask frame



FIG. 1 is a front perspective view of a mask frame according to my new ornamental design;

FIG. 2 is a front view thereof;

FIG. 3 is a rear view thereof; and,

FIG. 4 is a right side view thereof with the left side view being a mirror image.

The broken lines showing environment or structure of the mask frame are for illustrative purposes only and form no part of the claimed design.

The dotted lines represent the border between claimed and non-claimed subject matter of the mask frame and form no part of the claimed design. 

CLAIM The ornamental design for a mask frame, as shown and described. 